The present invention relates to a method of extruding and shaping thermoplastic material comprising the steps of continuously extruding a constant amount of molten thermoplastic material from a screw extruder, and continuously feeding a constant amount of the molten thermoplastic material extruded from the extruder to a die by means of a metering pump.
Molten thermoplastic material may come out of the extruder in varying amounts and cause what is known as surging owing to variations in plasticity of the material, the internal pressure of the extruder and the like. The above method is intended to check such a phenomenon affecting the shape and size of products extruded from the die by utilizing the measuring pump. A method of this nature proposed in the past is illustrated in FIG. 5, according to which pressure difference between positions upstream and downstream of a metering pump 101 is detected by sensors 102a, 102b and then the rotational frequency of a screw 103a of an extruder 103 is adjusted by a control device 104 to diminish deviation of the detected pressure difference from a predetermined value. However, a change of the rotational frequency of the screw will change the temperature within the extruder 103 which results in a change of the melting temperature of the plastic material for example. These latter changes can occur without any fixed relationship with the change of the rotational frequency of the screw 103a, and as a result make the pressure difference across the measuring pump 101 unstable and greatly deviated from the predetermined value. Therefore, from the point of view of control technique it is extremely difficult or impossible to provide a sufficient checking of surging appearing on products dispensed from a die 105 to meet practical requirements. Incidentally, FIG. 5 includes a motor 106 for driving the screw which is controlled by a control device 104 and a feeder 107 for feeding the thermoplastic material.